This invention relates to the fabrication of a structure by brazing from several tubes.
The production joining of small-sized mechanical elements can pose challenging problems. In an example, it is necessary to join three separate gas tubes during the construction of a Joule-Thomson cryostat. An inner tube lies concentrically within two end-to-end outer tubes. The tubes are joined together to form the multi-tubular structure. The brazing operation must form a gas-tight seal between the inner tube and the outer tubes, and must join the outer tubes in an end-to-end fashion. In existing practice, the brazing is performed in multiple steps by a highly skilled brazing torch operator.
Brazing is normally conducted using a flux to remove oxide that is present on the surfaces to be joined and to prevent the formation of additional oxides during the brazing operation. The braze metal is melted in the presence of the flux, wetted to the surfaces of the tubes, and then solidified to join the tubes. The flux is thereafter removed.
Brazing works well in many situations, but has limitations. In the case of the joining of three tubes for the Joule-Thomson cryostat, in one instance the outer diameter of the inner tube is about 0.019 inch, and the inner diameter of the outer tubes is about 0.022 inch. Because of the tight fit between the inner tube and the outer tube and the fact that the brazing produces a blind-end annular space between the tubes, it is virtually impossible to remove all of the flux from the annular space after brazing is complete.
There is a need for another approach to forming the multi-tubular structure for the Joule-Thomson cryostat, as well as for similar applications that arise in other production operations. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method of fabricating a multi-tubular structure that is structurally sound. The method uses brazing, but avoids the use of a brazing flux. It is therefore not necessary to remove any brazing flux from the tight space between the inner tube and the outer tubes at the completion of the brazing operation. The brazing of the three tubes together is accomplished in a single step rather than the multi-step process previously used. It is accomplished in a controlled fashion using specially designed tooling, and requires less skill in the personnel performing the brazing. The present approach also reduces the fabrication time and cost, and is more reproducible.
In accordance with the invention, a method of fabricating a multi-tubular structure without the use of a brazing flux comprises the steps of providing an inner tube, providing a bottom outer tube having a flared upper end, providing a top outer tube, and providing a mass of braze material having a braze-material melting point. A brazing assembly having no flux is assembled with the inner tube, the bottom outer tube overlying the inner tube, and the top outer tube overlying the inner tube. The top outer tube and the bottom outer tube are coaxial along a tube axis but longitudinally spaced apart along the tube axis. In the brazing assembly, the mass of braze material overlies the inner tube and resides within the flared upper end of the bottom outer tube. The method further includes brazing the brazing assembly in a vacuum. The step of brazing includes the steps of heating the braze material to a temperature above the braze-material melting point, and simultaneously moving the top outer tube and the bottom outer tube together along the tube axis so that their facing ends lie adjacent to each other. The heating is preferably performed inductively.
In the case where the inner tube, the bottom outer tube, and the bottom outer tube are portions of a cryostat, these elements are each made of a material selected from the group consisting of stainless steel and a copper-nickel alloy. The braze material is desirably an alloy of gold and nickel for these materials of construction. In the cryostat assembly of most interest, a difference between an outer diameter of the inner tube and an inner diameter of the top outer tube is less than about 0.004 inch.
The mass of braze material preferably comprises at least one ring of the braze material. More preferably, the mass of braze material comprises at least one ring of the braze material having an inner ring diameter of about the outer diameter of the inner tube. Yet more preferably, the mass of braze material comprises a first ring of the braze material having an inner first ring diameter of about the outer diameter of the inner tube, the first ring of the braze material residing within the flare of the bottom outer tube after the step of assembling; and a second ring of the braze material having an inner second ring diameter of about the outer diameter of the top outer tube and overlying a lower portion of the top outer tube after the step of assembling. Most preferably, the mass of braze material comprises a first ring of the braze material having an inner first ring diameter of about the outer diameter of the inner tube, the first ring of the braze material residing within the flare of the bottom outer tube after the step of assembling; a second ring of the braze material having an inner second ring diameter of about the outer diameter of the top outer tube and overlying a lower portion of the top outer tube after the step of assembling; and a third ring of the braze material having an inner third ring diameter of about the outer diameter of the inner tube, the third ring of the braze material residing within the flare of the bottom outer tube after the step of assembling.
The present approach desirably uses a brazing fixture that holds the inner tube stationary, holds one of the outer tubes (e.g., the bottom outer tube) stationary, and allows the other of the outer tubes (e.g., the top outer tube) to move vertically downwardly. The vertical downward motion brings the lower end of the top outer tube into abutting end-to-end contact with the flared end of the bottom outer tube, with the braze metal lying between the top outer tube and the bottom outer tube. Upon melting of the braze metal in vacuum, the molten braze metal wets the adjacent ends of the outer tubes and also the radially inwardly portion of the inner tube. Upon cooling, the three tubes are joined together.
This process requires attention to alignment and set-up, but the actual brazing is conducted by lower-skilled personnel than required for other brazing approaches. The result is a sound, reproducible braze without any flux and without the need to later clean away residual flux from a tightly confined annular volume. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.